I think what happens in your case, that the application of a high load, which initially caused an abrasive wear, further causes a smoothing of the sliding surfaces. As the attached surfaces become smoother, the tendency to adhesive wear increases.

Dear, hemant,

Can you specify roughness of both surface,hardness of both surface,

This is possible in general. In your case the higher load the higher nominal surface of the contact, and as a result lower wear rate.

Dear, Bhaskar

EN31 (counterface) -61 HRC

Copper Disc- 60 HV.

Is there any reference which does agree with the result.

This depends on the type of loads,if your load is like speed it will decrease over time, if it is like pressure or weight the wear rate will increase.

In general increases

it depends on the roughness and the amplitude of the displacement

I think there are several events that could lead to the variation of friction and wear.

1st surface roughness play a role specially in ball on disk experiments. when your surface is very smooth, this will lead to less abrasive wear and the temperature play a small role in establishing a solid tribofilm. Load increases will result in adhesive wear and the surface will be smoother and thus the wear rate will decrease. If the surface is around 0.3Ra I noticed from my experience that a solid tribofilm will attach itself to the substrate at high temperature and the wear rate will become minimum and that what we called the healing process that results specially in boundary lubrication under high load.

Rather than load what is more relevant is contact pressure. The contact area may increase due to wear of the ball and this as a consequence decreases the the contact pressure.

Alternatively it might be that both loads and contact pressure are increasing but wear dimities due to adhesive wear as already mentioned... basically if the contact pressure is high enough, the wear particles are "welded" back to the disc or ball rather than being removed from the contact area.

But we should not forget that if we have wear debris coming from abrasive wear we could have point contact and sharp increase in pressure if there is no tribofilm formation

The area of contact from ball over plane, initially must induce a high hertzian pressure, which result to a high wear rate. With the evolution of the test occurs an increase of the area of contact with the consequent reduction of the hertzian pressure, which leads to a lower rate of wear.

This behavior was verified with the tribological pair AISI 316 and UHMWPE.

See article: Oscillatory device for use with linear tribometer, for tribological evaluation of biomaterials, Review of Scientific Instruments 88, 075004 (2017).

Again if you have small precipitate wear debris they will contribute to the increased pressure unless you establish a healing area with solid tribofilm formation

Hemant Nautiyal

The phenomena you observed is the same for all tribology processes such as CMP processes for semiconductor and wire saw cutting for semiconductor and solar wafering. As the work production rate increases for the abrasion process so does the heat production rate, until you reach a point where no additional increment of the flux of useful work can be put through the fixed surface area and this is where work production is maximum Then the next increment of energy input (your load) can only go toward increasing the heat. This last increment is when work production is already at its maximum, so the next increment increases the entropy production rate so ensures that the useful work production must decline.

If you really need an increased abrasion rate, the trick is to add a chemical etchant which supplements the removal rate. This type of supplemental chemical etching is used in both CMP and plasma etching for semiconductor manufacturing.

again all depend on the materials and the tribofilm formation

Hi dear Hemnat,

In my opinion you should evaluate the surface of your counter part. your SEM images showed abrasive wear, so it could lead to formation of a tribolayer on the counterpart. this tribolayer could be rich in your studied part elements and oxide. this tribolayer protect your studied specimen from more surface destruction.

I recommend you to characterize both part of wear test; disk and ball in your studies. Its so important.

Good luck

There is an optimum load for every application depending on the variables you are using. It is not automatic

The coefficient of friction decreases with increasing contact load. This might be due to the severity of the contact pressure over the asperities tips, which could be enormous. At the same time, some of the asperities may fracture or crush, which results in the reduction of the surface roughness for the rubbing surfaces.

Besides, this reduction in wear rate can be associated with the formation of fine debris particles at the higher load to participate in the tribo-layer formed on the rubbing surfaces. Also, the frictional heating of asperities can also stimulate the tribo-oxide film formation.

All this depends on the tribofilm formation

Increasing contact pressure due to increasing applied load contributes to the conversion of retained wear debris on the wear surface which consequently reduces the wear rate

Solid Tribofilm formation will control the wear rate when load is increased. If there is no tribofilm then your wear rate is highly prone to increase

Definitely, the tribo film formation on the wear surface controls the wear rate at increasing applied load

The friction coefficient decreases with increasing normal load may be due to the fracture of some of the asperities, which decreases surface roughness for the counter surfaces.

It depends on many factors, I do not think there's a general answer to your question of yes/no.

I hope our paper below provides some insight.

Article The role of microstructure on wear mechanisms and anisotropy...

hi dear Hemnat,

In my opinion you should clean the wear surface

I think wear rate decreases initially due to formation of tribofilm.

Thanks for your valuable suggestions and explanations. I have conducted a detailed study on this aspect in the paper published in the journal mentioned below:

https://iopscience.iop.org/article/10.1088/2053-1591/ab28a2/pdf

Tribofilm bformation I’ll decrease wear but all depend on temperatures and not on the frictional variations once the tribofilm is formed

At high load value temperature of specimen are high, at high temperature metal specimen (Cu,Al etc) form metal oxide film on the meeting surface. Which hard enough and decrease the wear rate.

Here is the link to "Effect of cooling slope casting and partial remelting treatment on microstructure and mechanical properties of A319-xMg2Si In-Situ composites": https://www.sciencedirect.com/science/article/pii/S0921509320308649

Hemant Nautiyal , I think there is a general answer to your question. For any work production rate, any thermodynamic system can be considered to increase its rate of power (i.e. work production rate) as the input energy to the system rises. That is, input energy to the system converts to work production and heat production on the domain of the system. It does not matter if it is tribological or heat energy input into a power plant where hot gas is delivered to the surface of an electric generation turbine. We come to a critical point of power where the work production rate cannot increase further due to the finite amount of surface area across which the work energy flux is occurring. Once this critical point is reached any additional energy input deltaE'(input) can only go into the heat generation, deltaQ' on the domain and none of the next increment in input energy goes into more work production rate, deltaW' on the domain. Our company is publishing our mathematics on this, and we discovered it having analyzed wire sawing of silicon wafers. E'(input) = Q'(on the domain) + W'(on the domain). Then the solution is not an exponential, asymptotic solution of a DE. Instead, dW'/dQ' = 0 at the critical point on the domain. You simply have reached a point where work energy flux through the finite surface has reached its limit.

https://iopscience.iop.org/article/10.1088/1757-899X/1059/1/012020/meta

To understand if the wear rate decreases with increasing load, and from the description of your question, you need to further characterize the wear scar of your samples. You should also perform EDS analysis of the wear debris and compare the chemical composition of the wear debris to that of the sample. You can then confirm the origin of the debris. You can also evaluate the oxide level of the sample before and after the wear test, and compare it to the oxide level of the wear debris. Another piece of information that can help you understand these phenomena is to compare the microhardness of the surface with that of the wear debris. All of this information is also helpful in getting the best possible interpretation of the wear performance of your samples.

Therefore, if the samples appear to be more oxidized after the wear test, this indicate that a compact oxide tribological layer and also leads to a lower wear rate as the normal applied load increases.

https://iopscience.iop.org/article/10.1088/1757-899X/1059/1/012020/meta

You're experiment has gone past a thermodynamic 'critical point' (at real rates of power.....see www.realratethermo.com) which is the point at which no further work energy of abrasion can pass through the working surface.

So we have E' = Q' + W', so that the rate of input energy is equal to the heat production rate loss plus the work production rate of abrasion at the surface.

But we reach a critical point where dW'/dQ' = zero and the heat capacity and work capacity of the system are at 100%......this is the critical point for thermo at real rates of power and no further work production energy can pass through the finite surface area. Then any additional input energy, deltaE' will be converted entirely to heat production rate and the process rate (the useful work production rate) will degrade and not improve.

I have a webinar on this exact issue tomorrow, Wednesday at 19:00 (GMT-5). Please just sign up for the zoom call. There will be another webinar this month at a more convenient time for you. Leave me your email and I will add you to the invite list.

You'll also find this answer (i.e. explanation of the critical point) by going to the website and reading through the 'Guided Tour' that explains the derivation of the mathematics needed. You don't need any special background except differential equations.

www.realratethermo.com

Kindly refer this paper.

Article Parameter optimization for enhancing tribological properties...